This application is based on and incorporates herein by reference Japanese Patent Application No. 11-316290 filed Nov. 8, 1999.
The present invention relates to a deterioration detecting apparatus and method for detecting deterioration of an engine exhaust gas purifying device such as a catalyst converter based on the output of an oxygen sensor or an air/fuel ratio sensor disposed downstream the exhaust gas purifying device.
In recent engine exhaust gas purifying systems of automotive vehicles, an air/fuel ratio sensor or an oxygen sensor is disposed upstream and downstream an engine exhaust gas purifying catalytic converter. The upstream sensor is used to feedback-control the air/fuel ratio in the exhaust gas to an air/fuel ratio target, and the upstream sensor and the downstream sensor are used to detect the deterioration of the catalyst based on the ratio of frequencies and/or amplitudes of the outputs of the sensors.
The air/fuel ratio feedback control operates to alternately increase and decrease the feedback correction value in a generally periodic manner during the feedback control operation, so that the output of the upstream sensor responsively changes to a rich air/fuel ratio side and a lean air/fuel ratio side crossing the air/fuel ratio target. As the output of the upstream sensor also varies with engine operating conditions, the output of the downstream sensor also changes responsively. It is required for this reason to maintain the catalyst deterioration detecting operation to be less influenced by the changes in the output of the upstream sensor due to the changes in the operating conditions of the engine.
U.S. Pat. No. 5,220,788 (JP-A-5-10182) proposes to detect the deterioration of the exhaust gas purifying catalyst based on the output of the downstream sensor during a catalyst deterioration detecting period by forcibly modifying the changing period of the feedback correction value. However, the changing period of the air/fuel ratio feedback correction value should be set to maximize the exhaust purifying efficiency of the catalyst during the normal engine operating conditions. Thus, forced modification of the feedback correction value is likely to lower the accuracy of the air/fuel ratio feedback control and the exhaust gas purification efficiency during the normal operation conditions.
It is therefore an object of the present invention to provide a deterioration detecting apparatus and method for engine exhaust gas purifying devices, which is capable of detecting the deterioration of the purifying device accurately without lowering the exhaust gas purifying efficiency of the purifying device.
According to the present invention, oxygen responsive upstream and downstream sensors are disposed upstream and downstream a catalyst. An air/fuel ratio feedback control is effected to control an air/fuel ratio in the exhaust to an air/fuel ratio target based on an output of the upstream sensor. A sub-feedback control is effected to control the air/fuel ratio target based on an output of the downstream sensor. Deterioration of the catalyst is detected based on at least the output of the downstream sensor. A feedback gain of the sub-feedback control is changed during a catalyst deterioration detecting operation so that the downstream sensor exhibits larger changes in its output as the catalyst deteriorates more.
Preferably, the gain change is delayed for a predetermined period after a catalyst deterioration detecting condition is satisfied so that a fuel cut-off operation or fuel increase operation will not influence the deterioration detecting operation. Further, the amount of change in the feedback gain of the sub-feedback control is varied based on operating conditions of the engine.